Nickel-chromium alloy



June 18, 1963 Filed March 14, 1961 FIGJ FIG.2

ELONGATION- A. w. FRANKLIN ETAL 3,094,414

NICKEL-CHROMIUM ALLOY 2 Sheets-Sheet 1 i I o 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

TITANIUM/ALUMINIUM RATIO I4 (1.5%AU

k (2.0%Al) TITANIUM +ALUMINIUM CONTENT% ARTHUR W. FRANKLIN RONALD A. SMITH EDWARD G. RIC HA RDS INVENTORS C} ATTORNE Y June 1963 A. w. FRANKLIN ETAL 3,0

NICKELCHROMIUM ALLOY Filed March 14, 1961 2 Sheets-Sheet 2 20- 5 I5- 1o I (D S 1 Lu 20 600 700 800 900 TEMPERATUREC F!G.3

20-- I5 I :0- q (I) S 4 Lu I n I TEMPERATURE C FIG. 4

ARTHURWFRANKLIN RONALD ASMITH EDWARD G. RICHARDS INVENTORS "mam ATTORNEY United States Patent Office Patented June 18, 1963 3,094,414 NICKEL-CHRQMIUM ALLOY Arthur W. Franklin, Birmingham, Ronald A. Smith, West Hagley, and Edward G. Richards, Birmingham, England, assignors to The International Nickel Company, Inc., New York, N.Y., a corporation of Delaware Filed Mar. 14, 1961, Ser. No. 95,740 Claims priority, application Great Britain Mar. 15, 1960 9 Claims. (Cl. 75-171) The present invention relates to nickel-chromium alloys and, more particularly, to weldable nickel-chromium alloys including nickel-chromium-iron alloys.

Nickel-chromium and nickel-chromium-iron base heatand creep-resistant alloys containing titanium and aluminum to provide a precipitable phase of the Ni (Ti, Al) type and also containing molybdenum are now well known. In most alloys of this type, however, it is found that the ductility decreases with increasing temperature and is generally at a minimum in the temperature range 700 C.-850 C. While these alloys in the form of thin sheet not more than about A; inch thick can be welded under mild conditions and Without restraint, the ductility of the welded joints decreases to an even'greatcr extent in this temperature range, so that the'elongation of the welded joints in high-temperature tensile tests may fall below the commercially desirable minimum of or 7% To overcome this loss of ductility it has hitherto been necessary to apply high-temperature heat treatments after welding. The difiieulty in fabricating components by welding is particularly acute when the alloys are in sheet form and are used to make components such as jet-pipes for aircraft gas turbines, since at the high temperatures that must be used for the heat treatments the components tend to collapse or distort. Moreover, it is often diflicult or impossible to apply a post-weld heat treatment when the components are repaired in service under conditions where large-scale heat-treatment facilities are not available. Although attempts were made to overcome the foregoing diflicultics and other disadvantages, none, as far as we are aware, was entirely successful when carried into practice commercially on an industrial scale.

It has now been discovered that by means of a specially restricted alloy composition it is now possible to provide alloy structural elements which are readily weldable.

It is an object of the present invention to provide a novel weldable alloy. I

Another object of the invention is to provide a novel Welding process.

The invention also contemplates providing novel welded structures.

Other objects and advantages will become apparent from the following description taken in conjunction with the accompanying drawing in which:

FIGURE 1 is a graph illustrating the relationship between titanium and aluminum contents which are characteristic of alloys in accordance with the present invention;

FIG. 2 graphically depicts the effect of aluminum and titanium contents on the elongation of alloys;

FIG. 3 shows graphically the elongation characteristics at various temperatures of welded alloys set forth in the specification; and

FIG. 4 graphically illustrates the elongation characteristics at various temperatures of unwelded alloys set forth in the specification.

Generally speaking, the present invention contemplates providing heatand creep-resistant alloys that largely retain their strength and ductility after welding with a simple or no post-weld heat treatment. This object is achieved by adding critical amounts of the elements carhon, titanium, aluminum, molybdenum, boron and zirconium to nickel-chromium or nickel-chromium-iron base alloys containing about 20% chromium.

Alloys according to the invention contain in percent 5 by weight chromium about 15% to 25% and preferably "16% to20%, iron 0% to %,gcarbon about 0.04% to 0.15%, titanium about 0.7% to'2.5%, aluminum about 0.7% to 1.5%, molybdenum about 3% to 6%, and preferably 3.5% to 4.5%, boron 0.01% to-0.009%, zirconium 0.01% to 0.1%, silicon 0% to about 0.5%, manganese 0% to about 0.5%, cob-alt 0% to about 1%, "the balance (apart from impurities in an amount not exceeding 0.5%) being nickel in an amount not less than 35%. In addition, the total titanium and aluminum content is 2% to 3.5%, the ratio titanium/aluminum is 0.5 to 4 and the 'Ti-I-Al content is so correlated with the Ti/Al ratio that "thealloy lies within the area ABDA in FIGURE 1 of the accompanying drawing.

It is found that within these preferred ranges of com- 20 position little or no formation of sigma phase occurs on welding upon exposure in service totemperatures of 500 C.-800 C. for times up to. about 1000 hours.

' 'Theexact proportions of eachjofthe added elements vis of great importance in achieving satisfactory proper- 25 ties in the welded alloy,

' The carbon'content should beat le.ast'0.04% in' order 'to obtain an acceptable level of ductility. However, at carbon contents higher than'0.l5%, freecarbides are formed in the alloy duringworlcing that reducethe creep resistance at elevated temperatures and also make it chili cult to form the alloy into sheet or shape the sheetinto components. Prolonged. exposure to temperatures above 600 C. leads to precipitation of carbides at carbon contents of less than*0.15'%, and at carbon contents of above 0.08% these carbides tend to. be deposited at the grain boundaries in a form that may ultimately lead to embrittlement of the alloys, after say .500 hours or more. This effect is particularly. pronounced at temperatures of less than 700C. For applications involving very long periods of heating at such temperatures carbon contents in the range of 0.05% to 0.08% are therefore preferred. On the other hand for short-time applications in which to 0.12% may advantageously be used. the greatest ductility is required theacarbon contents up Both the total content of titanium and aluminum and the Ti/Al ratio are important. The elfect of'increasing Zith' Tia-Al contentis to increase strength of the alloy andmti the same time, decreasejtsiensile. ductility, both :at room'itemperature and at'elevated temperatures. At

Ti-FAI contents below 2%. the strength is inadequate,

'uvhiie above ;aTi+Al content depending on the Ti/Al *tmtio and defined by theline A-.-B in FIGURE 1 of the trlrawings the. alloys have: very low ductilities :at elevated temperatures, .particulafly after welding. Alloys. having :li-i-Al contents wrncsponding'rto pointsabove the line A A-z-B but lessrthan 3.5% havanndcs nably low' ductility at high temperatures afterdweldingzandat' Ti+Al contents greater than 3.5% their room temperatureiductility anlso' rapidly falls off to such an extent. that fabrication 60. .of thealloyin theiform of sheet becomes impractical.

-For applications where the greatest. duotility'is required and strengthis ofrather less importance, it is advantageous to make use of alloys the area EFCDE. The optimum comhination'of strength and. ductility is 5 obtained withiuthe area ABEEA at Ti+Al contents below 3%, while if the highest strength is required atthe expense of some sacrifice inductility alloys the portion. ofthe area ABFEA'that corresponds :to Ti-iiAI contents above 3% "should he used.

The effect of-inereasing the ratio of alumimum at a given Ti-iAi content is to increesethe ductility at elevated temperatures, as is shown in the curves in the graph forming FIG. 2 of the accompanying drawing. Referring now thereto, the curves marked 11, 12 and 13 show the variation in percentage elongation to fracture at 750 C. of alloys of the nominal composition: 20% Cr, 0.11% C, 0.003% B, 0.05% Zr, 3% Mo, 0.5% Si, 0.5% Mn, balance Ni, containing different amounts of titanium and aluminum. Curve 11 referes to alloys containing 1% Al, curve 12 to alloys containing 1.5% A1 and curve 13 to alloys containing 2% Al. These curves also demonstrate the decrease in high-temperature ductility that occurs as the total Ti+Al content is increased at all Ti/Al ratios.

The efiect of increasing the iron content of the alloys within the broad range to 45% is to increase the high-temperature ductility. This effect is shown by comparing the curve 12 in FIG. 2 of the drawing with the curve 14, which shows the variation of the elongation at 750 C. with total Ti+Al content of alloys nominally containing 1.5% A1 together with 18% Cr, 0.11% C, 37% Ni, 3% Mo, 0.003% B, 0.05% Zr, 0.5% Si, 0.5% Mn, balance Fe. Curve 14 also shows that the ductility of the iron-containing alloys falls with increasing Ti+Al content in the same way as for the substantially iron-free alloys of curve 12. All four of the curves 11 to 14 refer to alloys that had been heat treated by heating at 1100 C. for 8 minutes, cooling 'in air and finally ageing for 4 hours at 750 C.

The efiect upon the room-temperature ductility of varying the Ti+Al content and the Ti/Al are shown by the results in Table I. These results were obtained from tensile tests made at room temperature on bar samples of alloys of base composition: 0.08% C, 18% Cr, 5% Mo, 37% Ni, 0.003% B, 0.005% Zr, balance (apart from Ti and Al) substantially all Fe.

The bars were heat-treated before testing by heating for 30 minutes at 1150" C. air cooling, heating for 16 hours at 850 C. and again air cooling.

Replacement of nickel iron does, however, result in some decrease in the tensile strength at high temperatures and alloys with a low iron content for example 0% to 5% of iron, are therefore advantageous for applications where the highest tensile strengths are required. Alloys with say 30% to 40% iron, on the other hand, can be used where the highest duetilities are required but the tensile strength is not of such great importance.

It is important that the nickel content should not be reduced below 35%, otherwise severe embrittlement occurs after long exposure to temperatures in the range 650 C.850 C.

The presence of molybdenum improves the creep resistance of the alloys at temperatures and for this purpose at least 3% molybdenum should be present. 0n the other hand, the molybdenum content must not exceed 6%, as at higher levels the corrosion resistance becomes seriously impaired and forging is almost impossible. Within the range 3% to 6% the effect of molybdenum on the high-temperature strength of the alloys is complementary to that of Ti+Al. At a given Ti+A1 content, increasing the molybdenum content increases the hightemperature strength and the fall in strength that occurs on reducing the Ti+Al content from a given level can be partly or wholly compensated by increasing the molybdenum content by 0.5% for every 0.1% that the Ti+Al content decreases.

We have surprisingly found that very small amounts of boron improve the tensile ductility of the alloys at.

high temperatures and a content of boron within the very narrow range 0.001% to 0.009% is essential if welded 5 joints formed in the alloys are to have adequate ductility at elevated temperatures and are to be satisfactorily hotworkable. At boron contents below 0.001% the ductility of the alloys after welding is low and so is the stressrupture life. On the other hand if the boron content is in 10 creased above 0.009% the alloys tend to crack on welding, particularly in thick sections exceeding say inch, and preferably it does not exceed 0.004%. A particularly suitable content is 0.003%. The effect of the boron content on the ductility of welded joints at temperatures between 600 C. and 800 C. is shown by the following comparative figures for alloys of the nominal composition (apart from boron): 0.05% C, Cr, 5% M0, 2.6% (Ti+Al), Ti/Al 1.5, Zr 0.02%, Ni balance. The tests were carried out on sheet specimens 0.048 inch thick butt-welded without restraint by the argon-arc process.

Zirconium contributes to the creep resistance of the alloys and for this purpose it should be present in the range 0.01% to 0.1%, preferably 0.05%. Greater amounts of zirconium than 0.1 again may lead to cracking on welding.

The alloys may be melted in any convenient manner. It is common practice in air-melting nickel-chromium alloys to dcoxidize them by an addition of calcium or magnesium. When this technique is used for the present alloys, particularly those containing less than 5% iron, care should be taken that the residual calcium or magnesium content is as low as possible, preferably not exceeding 0.005%, as otherwise cracking may occur on welding thick sections or under restrained conditions, and the high-temperature ductility of the welded joints may be impaired.

To develop the optimum properties in the alloys, it is essential that they should be given an age hardening heat treatment consisting of solution heating followed by ageing at a lower temperature. Advantageously, this treatment consists of heating at 1020" C.1150 C., followed by cooling in air and ageing at 650 C.-850 C. For thin sections, say less than inch, the first treatment should be for 2-30 minutes and the second treatment for 2-16 hours. For thicker sections the first treatment should be for a longer time, say 2-8 hours. All the statements in this specification about the properties of the alloys refer to specimens that have been age hardened in this way. It is to be observed that welding" as employed in this specification refers to a process wherein elements, structures and the like are joined by means of a fusion process during which a liquid metallic contact is established while at least the adjacent surfaces of the structures, elements, etc., to be joined are at a temperature in excess of the incipient fusion temperature of the alloy or metal from which the structures or elements are made. Subsequent cooling during which contact is maintained establishes the weld bond by freezing of the liquid contacting metal.

If the alloys are to be welded, a solution heating should be applied before the welding operation is carried out but ageing before welding is unnecessary. After welding, no further solution heat treatment is in general necessary but where maximum strength is required, it

can be desirable to carry out a post-weld ageing treatment s consisting of heating for 2-l6 hours at 650 C. to'850" C. or 900 C. for preference in the higher part of this temperature range.

In the following table, alloys Nos. 1 to 3 are examples of alloys according to the invention, whereas alloys Nos. 4 to 6 are outside the invention.

Percent=peroent by weight.

Samples of each of the alloys in the form of sheet 0.048 inch thick were solution heated, alloys Nos. 1, 3, 4, 5 and 6 by heating at 1150 C. for 8 minutes and alloy No. 2 at 1020 C. for 20 minutes, and then autogenously buttwelded Without restraint by the argon-arc process and finally aged for 4 hours at 750 C. Table IV shows the strength and elongation of welded samples, tested with the weld at the center of the gauge length at a tempera ture of 750 C.

Table IV Alloy Nos Properties at r50 C.

1 i 2 1 3 4 I 5 I 6 U.T.S. (long tons/111. 37 36 33 32 41 41 Elongation (percent) 21 19 14 1.5 1. 3 4.6

The variation of the elongation with temperature of alloys Nos. 1, 2, 5 and 6 is shown in FIG. 3 of the accompanying drawing. Referring now thereto, this figure shows that whereas there is a very pronounced trough in the elongation temperature comes of alloys 5 and 6 in the temperature range 700 C.-900 C., which is responsible for their poor properties in the welded state, the alloys 1 and 2 of the invention experienced only a very slight drop in ductility.

As shown in FIG. 4, similar dilferences are present in the elongation temperature curves of the alloys in the unwelded state, though the fall in ductility with increases in temperature is not so great as in the welded alloys.

The molybdenum in the alloys can' be wholly or partly replaced by an equal atomic percentage of tungsten.

The alloys are suitable for use both in the sheet and wrought forms and can be used in the manufacture of welded structures, including composite welded structures comprising sheet components with wrought stiffening members Welded to them. Examples of such structures are parts of aircraft gas turbines such as jet pipes, flame tubes and jet silencers.

The alloys containing a high proportion of iron, for example, more than 30%, e.g., up to 40% iron, are resistant to corrosion in reducing atmospheres con-taining carbon and sulphur and are suitable for use in chemical plants where exposure to these conditions occurs.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily under stand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. An alloy consisting essentially by weight of about 15% to about 25% chromium, up to about 45% iron, about 0.04% to about 0.15% carbon, about 0.7% to 2.5% titanium, about 0.7% to 1.5% aluminum, an element selected from the group consisting of tungsten and molybdenum in an amount equivalent in atomic concentration in the alloy to about 3% to 6% molybdenum, 0.001% to 0.009% boron, 0.01% to 0.1% zirconium, up to about 0.5% silicon, up to about 0.5% manganese, up to about 1% cobalt with the balance being substantially nickel. in an amount not less than about 35%, the sum of said titanium percentage and said aluminum percentage being 2% to 3.5% and the ratio of said titanium percentage to said aluminum percentage being 0.5 to 4 and the said sum being correlated with said ratio so that when the values of said sum and ratio are platted, the resultant point lies within the area ABCDA set forth in FIGURE 1 of the accompanying drawing.

2. An alloy consisting essentially by weight of about 15% to about 25% chromium, up to about 45% iron, about 0.04% to about 0.15% carbon, about 0.7% to 2.5% titanium, about 0.7% to 1.5% aluminum, about 3% to 6% molybdenum, 0.001% to 0.009% boron, 0.01% to 0.1% zirconium, up to about 0.5% silicon, up to about 0.5% manganese, up to about 1% cobalt with the balance being substantially nickel, in an amount not less than about 35%, the sum of said titanium percentage and said aluminum percentage being 2% to 3.5% and the ratio of said titanium percentage to said aluminum percentage being 0.5 to 4 and the said sum being correlated with said ratio so that when the values of said sum and ratio are plotted, the resultant point lies within the area ABCDA set forth in FIGURE 1 of the accompanying drawing.

3. An alloy as set forth and defined in claim 2 wherein when the values of said ratio and sum are plotted, the resultant point lies within the area ABFEA set forth in FIGURE 1 of the accompanying drawing.

4. An alloy as set forth and defined in claim 2 wherein the chromium content is 17% to 20%.

5. An alloy as set forth and defined in claim 2 wherein the iron content is up to 5 6. An alloy as set forth and defined in claim 2 wherein the boron content is 0.001% to 0.004%.

7. An alloy as set forth and defined in claim 2 wherein the carbon content is 0.05% to 0.08%.

8. A welded structure of a kind subjected in use to prolonged stress in the temperature range 700 C. to 850 C. having at least one welded element made of an alloy consisting essentially by weight of about 15% to about 25% chromium, up to about 45% iron, about 0.04% to about 0.15% carbon, about 0.7% to 2.5% titanium, about 0.7 to 1.5% aluminum, about 3% to 6% molybdenum, 0.001% to 0.009% boron, 0.01% to 0.1% zirconium, up to about 0.5% silicon, up to about 0.5% manganese, up to about 1% cobalt with the balance being substantially nickel in an amount not less than 35%, the sum of said titanium percentage and said aluminum percentage being 2% to 3.5% and the ratio of said titanium percentage to said aluminum percentage being 0.5 to 4 and the said sum being correlated with said ratio so that when the values of said sum and ratio are plotted, the resultant point lies within the area ABCDA set forth in FIGURE 1 of the accompanying drawing.

9. A jet pipe for a gas turbine engine having a weld joint and at least one element made of an alloy consisting essentially by weight of about 15% to about 25% chromium, up 'to about 45% iron, about 0.04% to about 0.15% carbon, about 0.7% to 2.5% titanium, about 0.7% to 1.5% aluminum, about 3% to 6% molybdenum, 0.001% to 0.009% boron, 0.01% to 0.1% zirconium, up to about 0.5% silicon, up to about 0.5% manganese, up to about 1% cobalt with the balance being substantially nickel in an amount not less than 35%, the sum of said titanium percentage and said aluminum percentage being 2% to 3.5% and the ratio of said titanium percentage to said aluminum percentage being 0.5 to 4 and the said sum being correlated with said ratio so that when the values of said sum and ratio are plotted, the resultant point lies within the area ABCDA set forth in FIGURE 1 of the accompanying drawing.

References Cited in the file of this patent UNITED STATES PATENTS Pritchard et a1. Oct. 25, 1960 Bieber Mar. 28, 1961 FOREIGN PATENTS Great Britain Apr. 20, 1955 Canada Nov. 12, 1957 

1. AN ALLOY CONSISTING ESSENTIALLY BY WEIGHT OF ABOUT 15% TO ABOUT 25% CHROMIUM, UP TO ABOUT 45% IRON, ABOUT 0.04% TO ABOUT 0.15% CARBON, ABOUT 0.7% TO 2.5% TITANIUM, ABOUT 0.7% TO 1.5% ALUMINUM, AN ELEMENT SELECTED FROM THE GROUP CONSISTING OF TUNGSTEN AND MOLYBDENUM IN AN AMOUNT EQUIVALENT IN ATOMIC CONCENTRATION IN THE ALLOY TO ABOUT 3% TO 6% MOLYBDENUM, 0.001% TO 0.009% BRON, 00.01% OF 0.1% ZIRCONIUM, UP TO ABOUT 0.5% SILICON, UP TO ABOUT 0.5% MANGANESE, UP TO ABOUT 1% COBALT WITH THE BALANCE BEING SUBSTANTIALLY NICKEL, IN AN AMOUNT NOT LESS THAN ABOUT 35%, THE SUM OF SAID TITANIUM PERCENTAGE AND SAID ALUMINUM PERCENTAGE BEING 2% TO 3.5% AND THE RATIO OF SAID TITANIUM PERCENT TO SAID ALUMINUM PERCENTAGE BEING 0.5 TO 4 AND THE SAID SUM BEING CORRELATED WITH SAID RATIO SO THAT WHEN THE VALUES OF SAID SUM AND RATIO ARE PLATTED, THE RESULTANT POINT LIES WITHIN THE AREA ABCDA SET FORTH IN FIGURE 1 OF THE ACCOMPANY DRAWING. 